(1) Field of the Invention
The present invention relates to improvement of heavy duty low profile pneumatic radial tires.
(2) Related Art Statement
When the aspect ratio of a tire is lowered to decrease the dimension and weight thereof, the diameter of the belt of the tire increases and grows due to deformation caused by application of an internal pressure and creeps produced in running history. Consequently, it is likely that harmful deformation occurs at a shoulder portion and that separation occurs at an end portion of a belt layer.
In order to reduce such drawbacks, U.S. Pat. No. 4,387,758 proposes a technique to control the shape of a carcass or U.S. Pat. No. 4,420,025 discloses a technique to provide an auxiliary layer having cords arranged substantially in parallel with a tire circumferential direction.
However, in the case of the carcass shape proposed in U.S. Pat. No. 4,387,758, as the aspect ratio is decreased, an effect to suppress the deformation of the shoulder portion due to the application of the internal pressure becomes smaller. On the other hand, since an axial end portion of a belt layer is bent toward the tire inside relative to the carcass layer as a natural axis, an axial component of stress acts upon the belt layer. As a result, although the shoulder portion is not largely deformed due to the application of internal pressure, a circumferential component of the stress acts upon the belt layer. Thus, an effect to restrain enlargement of a tire diameter resulting from creep in the tire running history is small.
To the contrary, the technique as proposed in U.S. Pat. No. 4,420,025 in which the enlargement of the diameter of the shoulder portion is suppressed by locally increasing rigidity of the belt per unit width is effective to restrain the diameter enlargement due to the tire running history. However, this technique has the drawback that the tire is likely to be unevenly worn due to variation in rigidity occurring on the ground contact surface.